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A2 Biology Unit 5 page 2 Biology Unit 5 SpecificationControl Systems are substances that stimulate their target cells via the bloodOrganisms increase their chance of survival by responding system. This results in slow, long-lasting and widespreadto changes in their environment. responses. The second messenger model of adrenaline and glucagon action. Histamine and prostaglandins are localThe Nerve Impulse chemical mediators released by some mammalian cells thatThe structure of a myelinated motor neurone. The affect only cells in their immediate vicinity.…read more

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A2 Biology Unit 5 page 3Genetics · Small interfering RNA (siRNA) as a short, double-strandThe Genetic Code of RNA that interferes with the expression of a specificThe genetic code as base triplets in mRNA which code for gene. Interpret data provided from investigations intospecific amino acids. The genetic code is universal, non- gene expression.overlapping and degenerate. The structure of molecules ofmessenger RNA (mRNA) and transfer RNA (tRNA).…read more

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A2 Biology Unit 5 page 4 The Human Nervous SystemHumans, like all living organisms, can respond to changes in the environment and so increase survival.Humans have two control systems to do this: the nervous system and the endocrine (hormonal) system.We'll look at the endocrine system later, but first we'll look at the nervous system. The human nervoussystem controls everything from breathing and standing upright, to memory and intelligence.…read more

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A2 Biology Unit 5 page 5Nerve Cells dendritesThe nervous system composed of nerve cells, or neurones. A neurone has acell body with extensions leading off it. Several dendrons carry nerve impulses Sensory Neuronetowards the cell body, while a single long axon carries the nerve impulse away dendronfrom the cell body.…read more

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A2 Biology Unit 5 page 6The Nerve ImpulseNeurones transmit simple on/off signals called impulses (never talk about nerve signals or messages). Theseimpulses are due to events in the cell membrane, so to understand the nerve impulse we need to revisesome properties of cell membranes.The Membrane PotentialAll animal cell membranes contain a protein pump called the Na+K+ATPase. This uses the energy fromATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in.…read more

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A2 Biology Unit 5 page 7an electrical pulse at one end of an axon and measure the voltage changes at the other end, using anoscilloscope: stimulating recording stimulator oscilloscope electrodes electrodes squid giant axon isotonic bathThe normal membrane potential of these nerve cells is ­70mV (inside the axon), and since this potentialcan change in nerve cells it is called the resting potential. When a stimulating pulse was applied a briefreversal of the membrane potential, lasting about a millisecond, was recorded.…read more

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A2 Biology Unit 5 page 8How do Nerve Impulses Start?In the squid experiments the action potential was initiated by the stimulating electrodes. In living cells theyare started by receptor cells. These all contain special receptor proteins that sense the stimulus. Thereceptor proteins are sodium channels that are not voltage-gated, but instead are gated by the appropriatestimulus (directly or indirectly).…read more

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A2 Biology Unit 5 page 9How can Nerve Impulses convey strength?How do impulses convey the strength of the stimulus? Since nerve impulses are all-or-nothing, they cannotvary in size. Instead, the strength of stimulus is indicated by the frequency of nerve impulses. A weakstimulus (such as dim light, a quiet sound or gentle pressure) will cause a low frequency of nerve impulsesalong a sensory neurone (around 10Hz).…read more

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A2 Biology Unit 5 page 10SynapsesThe junction between two neurones is called a synapse. An action potential cannot cross the gap betweenthe neurones (called the synaptic cleft), and instead the nerve impulse is carried by chemicals calledneurotransmitters. These chemicals are made by the cell that is sending the impulse (the pre-synapticneurone) and stored in synaptic vesicles at the end of the axon.…read more